Linear distributed pixel differential amplifier having mirrored inputs

ABSTRACT

A pixel circuit that partially incorporates an associated column amplifier into the pixel circuitry. By incorporating part of a mirrored amplifier into the pixel, noise from the pixel is reduced.

FIELD OF THE INVENTION

The invention relates generally to digital image processing and moreparticularly to a method and apparatus for pixel readout.

BACKGROUND OF THE INVENTION

There is a current interest in CMOS active pixel imagers for possibleuse as low cost imaging devices. An exemplary pixel circuit of a CMOSactive pixel sensor (APS) is described below with reference to FIG. 1.Active pixel sensors can have one or more active transistors within thepixel, can be made compatible with CMOS technologies, and promise higherreadout rates compared to passive pixel sensors. FIG. 1 illustrates anexemplary pixel 4T cell 10 in an image sensor 5, where “4T” designatesthe use of four transistors to operate the pixel 10 as is commonlyunderstood in the art. A 4T pixel has a photodiode 12, a transfertransistor a reset transistor 13, a source follower transistor 14, and arow select transistor 15. It should be understood that FIG. 1 shows thecircuitry for the operation of a single pixel, and that in practical usethere will be an M-by-N array of identical pixels arranged in rows andcolumns with the pixels of the array accessed using row and columnselect circuitry, as described in more detail below.

The photodiode 12 converts incident photons to electrons that aretransferred to a storage node FD through transfer transistor 11. Asource follower transistor 14 has its gate connected to node FD andamplifies the signal appearing at node FD. When a particular rowcontaining pixel 10 is selected by the row select transistor 15, thesignal amplified by transistor 14 is passed to a column line 17 to thereadout circuitry. The photodiode 12 accumulates a photo-generatedcharge in a doped region of the substrate. It should be understood thatthe CMOS imager 5 might include a photogate or other photoconversiondevice, in lieu of a photodiode, for producing photo-generated charge.

A reset voltage source Vrst is selectively coupled through resettransistor 13 to node FD. The gate of transfer transistor 11 is coupledto a transfer control line which serves to control the transferoperation by which photodiode 12 is connected to node FD. The gate ofreset transistor 13 is coupled to a reset control line, which serves tocontrol the reset operation in which Vrst is connected to node FD. Therow select control line is typically coupled to all of the pixels of thesame row of the array. A supply voltage source is coupled to the sourcefollower transistor 14. Although not shown in FIG. 1, column line 17 iscoupled to all of the pixels of the same column of the array andtypically has a current sink transistor 16 at one end. The gate of rowselect transistor 15 is coupled to row select control line.

As known in the art, a value is read from pixel 5 using a two-stepprocess. During a reset period, node FD is reset by turning on resettransistor 13, and the reset voltage is applied to node FD and read outto column line 17 by the source follower transistor 14 (through theactivated row select transistor 15). During a charge integration periodthe photodiode 12 converts photons to electrons. After the integrationperiod the transfer transistor 11 is then activated, allowing theelectrons from photodiode 12 to collect at node FD. The charges at nodeFD are amplified by source follower transistor 14 and selectively passedto column line 17 by row access transistor 15. As a result, the twodifferent values—the reset voltage (Vrst) and the image signal voltage(Vsig)—are readout from the pixel 10 and sent by the column line 17 toreadout circuitry, where each voltage is sampled and held for furtherprocessing as known in the art.

All pixels in a row are read out simultaneously onto respective columnlines 17 and stored in respective sample and hold circuits. Then thecolumn circuitry in the sample and hold circuits are activated for resetand signal voltage readout processing.

Typically, pixel readout has been accomplished with source followertransistor 14 in the pixel 10, selectable by row select transistor 15.This source follower transistor 14 has a gain less then unity (˜0.8) dueto the finite output impedance of the source follower transistor 14 andcurrent sink transistor 16. In addition, the source follower transistor14 is typically an N channel transistor in a grounded substrate,resulting in a gain non-linearity over the signal range due to back-gatebias (also known as body effect or bulk effect). Process variations canalso cause a slightly different gain from one pixel to another. Thiscombined with the non-linearity can add additional kTC noise and PRNU(photo-response non-uniformity) to the image sensor 5. Finally, thesource follower settling time when selected can be quite prolonged dueto low loop-gain and small source follower gain. This long settling timecan both make the source follower transistor 14 subject to RTS (e.g.,1/f) noise as well as increase the row readout time causing lower framerates. The RTS noise is typically caused by traps in the oxide under thesource follower transistor 14. The RTS noise gets worse the longer thesource follower transistor is amplifying a signal. If the pixel outputsettling time can be reduced, the RTS noise will improve dramatically.

FIG. 2 shows a CMOS active pixel sensor integrated circuit chip 2 thatincludes an array of pixels 10 and a controller 23 that provides timingand control signals to enable reading out of signals stored in thepixels in a manner commonly known to those skilled in the art. Exemplaryarrays have dimensions of M-by-N pixels, with the size of the array 5depending on a particular application. The imager is read out a row at atime using a column parallel readout architecture. The controller 23selects a particular row of pixels in the array 5 by controlling theoperation of row addressing circuit 21—the vertical addressingcircuit—and row drivers 22. Charge signals stored in the selected row ofpixels are provided on the column lines 17 (FIG. 1) to a readout circuit25 in the manner described above. The pixel signals (reset voltage Vrstand image signal voltage Vsig) read from each of the columns can then beread out, sampled and held, subtracted (Vrst−Vsig) and the resultsequentially sent to further processing such as digitization, using acolumn addressing circuit. Differential pixel signals (Vrst, Vsig)corresponding to the readout reset signal (Vrst) and image signal (Vsig)are provided as respective outputs Vout1, Vout2 of the readout circuit25 for subtraction and subsequent processing. Alternatively, readoutcircuit 25 provides a combined differential signal of the two signalsVrst, Vsig.

As noted, the source follower transistor 14 limits output swingavailable for the pixel output signals, where the gain maybe limited to0.8 of the signal applied to input gate of the source followertransistor, it would be desirable to increase the gain applied to pixeloutput signals.

BRIEF SUMMARY OF THE INVENTION

The invention provides a new APS pixel architecture capable of providingan increased gain for pixel output signals by integrating the pixelarchitecture with a differential amplifier in the column readout signalprocessing path. An exemplary embodiment of the invention has the sourcefollower as part of an amplifier's differential input stage and has themajority of the differential amplifier reside as a column parallelcircuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages and features of the invention willbecome more apparent from the detailed description of exemplaryembodiments provided below with reference to the accompanying drawingsin which:

FIG. 1 is a block diagram of a conventional active pixel;

FIG. 2 is a block diagram of a conventional imaging chip;

FIG. 3 is a block diagram of a conventional folded cascode amplifier;

FIG. 4 is a pixel circuit in accordance with an exemplary embodiment ofthe invention;

FIG. 5 is a portion of pixel array in accordance with an exemplaryembodiment of the invention;

FIG. 6 is a portion of a column of pixel array and associated columnamplifier circuitry in accordance with an exemplary embodiment of theinvention;

FIG. 7 is a timing diagram showing a method of operating the circuits ofFIGS. 4-6;

FIG. 8 is a pixel circuit in accordance with another exemplaryembodiment of the invention; and

FIG. 9 is a block diagram representation of a processor-based systemincorporating a CMOS imaging device in accordance with an exemplaryembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings, which are a part of the specification, and inwhich is shown by way of illustration various embodiments whereby theinvention may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to make and use theinvention. It is to be understood that other embodiments may beutilized, and that structural, logical, and electrical changes, as wellas changes in the materials used, may be made without departing from thespirit and scope of the present invention.

Before describing the invention in detail, a brief description of aconventional differential amplifier is provided. FIG. 3 shows aconventional unity gain differential amplifier with differential inputstage comprised of input transistors 65 and 68, tail current transistor67, a load current 61, 62, and 63, cascode transistors 64, 69 and diodeconnected transistor 66, with circuits 61, 62 and 63 acting as currentmirrors. Transistors 64 and 65 comprise the positive input leg andtransistors 68 and 69 comprise the negative input leg. The outputs 72and 73 of the input stage are fed to a differential output amplifier 56.The output of amplifier 56 provides an output voltage at terminal 54.The output voltage at terminal 54 is fed back by line 74 as an input tothe gate of transistor 68. The negative feedback 74 in amplifier 56implements a closed loop unity gain buffer. The voltage on the ‘input’terminal 52 is buffered to output node ‘Output’ with a gain very closeto unity. Diode connected N channel transistor 66 is biased by currentmirror 62 to regulate a nearly constant Vds across input transistors 65and 68 though cascode transistors 64 and 69 over the entire input commonmode range. This provides improved CMRR (Common mode rejection ratio)which is directly related to unity gain buffer linearity. The regulatedcascode input stage also provides a higher gain in the input stageallowing a lower output stage gain, which is turn limits the elementscontributing to the entire amplifier input noise to only a fewtransistors. The output amplifier 56 can be any known amplifier outputstage. It should also be noted that either/both current mirror 61 or 63can be a diode connected with bias voltage being connected to either ofthe cascode transistor 64 or 69 drains depending on amplifier topologychosen.

The invention is now described with reference to FIGS. 4-9. FIG. 4 showsa portion of a pixel array 501 of an image processing system 500including a pixel 300. Pixel 300 includes photodiode 312, transfer gate301, floating diffusion region 303, and reset transistor 302. Inaddition, the pixel includes a differential input transistor 314, inputcascode transistor 305, cascode sample transistor 304 and cascodeholding node 313. Although described with respect to a photodiode, theinvention is not so limited and any similar element maybe used thatconverts energy received in photons to charges.

The photodiode 312 is coupled to the gate of the positive differentialinput transistor 314 through the transfer transistor 301. The floatingdiffusion region 303 is located between the positive differential inputtransistor 314 and the transfer transistor 301. The gate of the transfertransistor 301 is coupled to transfer signal line Tx_n 309. One of thesource/drain regions of the reset transistor 302 is coupled to thefloating diffusion region 303 and the other source/drain region of thereset transistor 302 is coupled to cascode signal line 306. The gate ofthe reset transistor 302 is coupled to reset signal line Rst_n 310.

One of the source/drain regions of the input cascode transistor 305 iscoupled to the positive differential input transistor 314 and the othersource/drain region of input cascode transistor 305 is coupled to loadline 307. The source/drain region of the positive differential inputtransistor 314 not coupled to the input cascode transistor 305 iscoupled to load line 308. One source/drain region of the cascode sampletransistor 304 is coupled to the gate of the input cascode transistor305 and the other of the source/drain regions of cascode sampletransistor 304 is coupled to cascode line 306. The gate of the cascodesample transistor 304 is coupled to the row select signal line Row_n311.

FIG. 5 depicts the pixel array 501 of FIG. 4 in the environment oflarger portion of a pixel array 501. FIG. 5 depicts four pixels 300, 300a, 300 b, and 300 c of array 501 as part of the image processing system500. As seen in FIG. 5, the pixels 300, 300 a, 300 b, and 300 c are partof an array 501 having M rows×N columns, although only four pixels areshown. There are arrayed transfer control lines (i.e., Tx_n), resetcontrol lines (i.e., Rst_n), and row select control lines (i.e., Row_n)routed along rows across all the columns of the array 501. For example,for the nth row, there is transfer control line Tx_n 309, reset controlline Rst_n 310, and row select control line Row_n 311. Additionally, forthe n+1 row, there is transfer control line Tx_n+1 389, reset controlline Rst_n+1 390, and row select control line Row_n+1 391. There arealso arrayed cascode control lines, top load lines, and bottom loadlines routed along columns across all the rows of the array 501. Forexample, for the m^(th) column, there is a cascode control line 306, topload line 307, and bottom load line 308. Additionally, for the m^(th)+1column, there is a cascode control line 386, top load line 387, andbottom load line 388. The top and bottom load lines are the negativeinput transistors distributed across the columns of the pixel array 501.

As seen in FIGS. 4 and 5, a pixel 300 is accessed for a reset (and areadout) operation by enabling the associated cascode control line andthe row line of the pixel and, ideally, disabling all other cascodecontrol lines and row lines. Prior to performing a readout operation, itis desirable to perform an extinguish operation to clear or neutralizethe pixel 300 from having any remaining signals in the pixel 300. Toaccess pixel 300 for a readout, cascode line 306 and Row_n 311 areenabled, for example, by a logic “high” signal. Operation of the pixel300 is described in greater detail below.

FIG. 6 is a schematic diagram of one pixel 300 of pixel array 501coupled to associated column amplifier circuitry in accordance with anexemplary embodiment of the invention. As seen in FIG. 6, the imageprocessing system 500 includes pixel array 501, a forcing signal circuit516, and a column amplifier 510. The pixel array 501 includes aplurality of rows and columns although only one pixel to be read out isshown. The pixel 300 shown in FIG. 6 is representational of one pixel inthe pixel array 501 (as described with reference to FIGS. 4 and 5)belonging to the currently selected row and located in the same columnas amplifier 510.

Column amplifier 510 includes a diode circuit 520, a negative input legcircuit 530, and a differential amplifier 540. Transistors containingdiode connected transistor 521, and pixel transistors 304, 305 and 314comprise the positive input leg for column amplifier 510. The negativeinput leg 530 of the column amplifier 510, is formed by transistors 532,533 and 534. Thus, the negative input leg 530 includes transistors 532,533, and 534 that complements positive input leg transistors 304, 305,and 314 in the pixel 300 of the same column. Tail current mirror 561provides bias current for both positive input leg 304, 305, 314 andnegative input leg 530.

The differential output amplifier 540 has two branches, one branch whichincludes transistors 541, 542, 543, and 544 and other branch whichincludes transistors 546, 547, 548, and 549. The gates of transistors541 and 546 are coupled to an external bias voltage bias3, 542 and 547are coupled to an external bias voltage bias4, 543 and 548 are coupledto an external bias voltage bias5, and 544 and 549 are connected as awide swing cascoded current mirror. The output stage is identical to thewell-known folded cascode amplifier topology. The input of one branch ofthe differential amplifier 540 is coupled to load line 307 and the inputof other branch of the differential amplifier 530 is coupled to loadline 308. The output of the differential amplifier 540 is provided bothon line 335 to a downstream circuit and is fed back into the negativedifferential input transistor 534 to form a closed loop unity gainamplifier.

The diode circuit 520 couples cascode line 306 to load line 308 througha contact transistor 579. The contact transistor 579 is selectivelyenabled when reading out signals from pixel 300 in the column;transistor 579 is otherwise disabled to reduced power loss through thecolumn amplifier 510 when either transistor in the forcing signalcircuit 516 is active. Circuit 560 selectively couples bottom load line308 to ground as controlled by bias1.

The forcing signal circuit 516 selectively controls coupling the cascodeline 306 to either ground, to the supply voltage or leaves the cascodeline 306 floating. The forcing signal circuit 516 includes an n-channeltransistor 514 and a p-channel transistor 512. Cascode line 306 iscoupled to a voltage source through transistor 512 and cascode line 306is coupled to ground through transistor 514. Transistor 512 is closed byproviding a logic low signal to its gate. Transistor 514 is closed byproviding a logic high signal to it.

The operation of imaging processing system 500 in reading out pixelreset Vrst and image Vsig signals is now described with reference to thesimplified signal timing diagram of FIG. 7.

Before performing a reset or readout from a pixel 300, it is imperativethat all cascode holding nodes 313 in all rows other than the currentlyselected rows are cleared of any residual signals and hold a voltageclose to ground. Only the currently selected rows cascode holding node313 is supposed to have a voltage significantly above ground.Accordingly, transistor 514 is activated by the Force low signal goinghigh to couple cascode line 306 to ground while the Row_n voltage 311 inall rows except the currently selected are set high to pass the groundfrom cascode line 306 into cascode holding nodes 313. During this timethe /Force High signal is high to keep transistor 512 turned off. TheRow_n signals 311 for each row except the currently selected rows arereturned to ground to sample-and-hold a voltage close to ground on allcascode holding nodes 313, independent of future cascode line 306voltages. A voltage close to ground on cascode holding node 313 willeffectively turn off cascode transistors 305 for all other rows than thecurrently selected rows and prevent any other row than the currentlyselected from influencing the readout.

In order to perform a reset of the pixel 300, the cascode signal line306 is set to a high signal by applying a low signal on the /Force Highinput causing p-channel transistor 512 to conduct. At this time theForce low signal is set low turning off transistor 514. As a result, asupply voltage is coupled through transistor 512 to cascode line 306.The reset control line connected to the gate of transistor 302 isenabled by Reset (n) going high, thereby temporarily activating resettransistor 302 and coupling the floating diffusion region 303 to asupply voltage through the cascode line 306. To read out the floatingdiffusion reset signal, the Row_n signal 311 is then set high, therebyactivating row transistor 304 and coupling the gate of input cascodetransistor 305 to cascode line 306. The cascode line 306 is at a logichigh so transistor 305 turns on, thereby coupling transistor 314 to theload line 307. The charge and related voltage on the floating diffusionregion 303 controls how much differential input transistor 314 turns onand therefore how much of the tail current 561 flows from the load 307to load 308 rather than through negative leg 530 and thereby affects thetwo inputs to the differential amplifier 540.

The linearity of the unity gain buffer in this mode of operation isbetter than a source follower, but still suffers due to poor CMRR as thetwo input stage cascode transistors 305 and 533 are forced high throughtransistor 512. To further improve linearity after the entire amplifierloop has settled, transistor 512 is turned off leaving the cascode line306 floating and contact transistor 579 is then enabled. Thisinstantiates a second regulation loop comprising load line 308, cascodediode 521, cascode line 306 through enabled cascode sample transistors304 and 532 into cascode transistors 305 and 533. This loop becomesidentical to diode coupled transistor 66 into cascode devices 64 and 69in the known input stage architecture 50. FIG. 3 and FIG. 7 therebyrelates in operation in this configuration by relating transistors 561to 67, 314 to 65, 305 to 64, 534 to 68, 533 to 69 and 521 to 66. As aresult, a reset signal is readout on line 335 from the pixel 300 througha column amplifier 510 to a downstream circuit.

The output on line 335 is fed back to transistor 534 which is in thenegative input leg of differential amplifier 540 formed by transistors534, 533, 532, all of which complement the positive input todifferential amplifier 540 formed by transistors 314, 305, 304, a unitygain from selected row floating diffusion 303 to output line 335 isachieved which is applied to the pixel signal, which at this point isthe reset signal Vrst. The Row_n signal 311 remains enabled, and theTx_n signal 309 is enabled thereby temporarily closing transfertransistor 301 and coupling the photodiode 312 to the floating diffusionregion 303 allowing the charge on the photodiode 312 to be transferredto the floating diffusion region 303. The charge on the floatingdiffusion region 303 controls how much differential input transistor 314turns on and therefore how much charge flows from the load 307 to loadline and affects the input of the amplifier 540. As a result, an imagesignal is readout from the pixel 300 through the column amplifier to adownstream circuit on line 335. As such, the feedback path from outputline 335 to transistor 534 sets a unity gain.

The output on line 335 can be fed to a conventional sample and holdcircuit which captures the reset Vrst and image Vsig signals. Byincorporating part of a column differential amplifier into the pixelcircuit the variances of the kTC noise and other noise sources common tothe two samples is reduced—a technique well known as CDS (correlateddouble sampling).

FIG. 8 is a pixel circuit in accordance with another exemplaryembodiment of the invention. The pixel circuit of FIG. 8 is similar tothe pixel circuit of FIG. 6 except that the pixel circuit of FIG. 8includes circuit 898 in the feedback path between output 335 andtransistor 534. As the amplifier circuit of FIG. 6 is set up as a unitygain circuit, it may be desirable an increase the gain of the signalprocessed beyond unity. Circuit 898 controls the amount of feedbackapplied to transistor 534. The amplifier gain can be set by adjustingthe values of the voltage dividing capacitors 892, 894. As isconventionally known, by varying the values of the capacitors in circuit898, the gain of amplifier circuit 810 can be adjusted. To properlyoperate, feedback capacitors in circuit 898 needs reset circuitry (notshown). It is possible to incorporate CDS by resetting the feedbackcapacitor 892 during Vrst readout.

The invention may be employed in the pixel array readout circuitdepicted in FIG. 2, which in turn may be coupled to a processing system.FIG. 9 shows a processing system 1100, which includes an imagingprocessing device 500 having the general construction of FIG. 2 andemploying the invention described with respect to FIGS. 4-8 as part ofthe readout circuit. The system 1100 is exemplary of a system havingdigital circuits that could include image sensor devices. Without beinglimiting, such a system could include a computer system, camera system,scanner, machine vision, vehicle navigation, video phone, surveillancesystem, auto focus system, star tracker system, motion detection system,image stabilization system, and other image acquisition or processingsystem.

System 1100, for example a camera system, generally comprises a centralprocessing unit (CPU) 1110, such as a microprocessor, that communicateswith an input/output (I/O) device 1150 over a bus 1170. Imagingprocessing device 500 also communicates with the CPU 1110 over the bus1170. The system 1100 also includes random access memory (RAM) 1160, andcan include removable memory 1130, such as flash memory, which alsocommunicate with the CPU 1110 over the bus 1170. The imaging processingdevice 100 may be combined with a processor, such as a CPU, digitalsignal processor, or microprocessor, with or without memory storage on asingle integrated circuit or on a different chip than the processor.

It should be appreciated that other embodiments of the invention includea method of manufacturing the system 1100. For example, in one exemplaryembodiment, a method of manufacturing a CMOS readout circuit includesthe steps of fabricating, over a portion of a substrate an integratedsingle integrated circuit, at least an image sensor with a readoutcircuit as described above with respect to FIGS. 4-9 using knownsemiconductor fabrication techniques.

While the invention has been described and illustrated with reference tospecific exemplary embodiments, it should be understood that manymodifications and substitutions could be made without departing from thespirit and scope of the invention. Accordingly, the invention is not tobe considered as limited by the foregoing description but is onlylimited by the scope of the claims.

1. An imaging system, comprising: a pixel array, comprising an imagerreadout circuit, comprising: a first circuit provided in at least onepixel of said pixel array for providing a pixel output; a differentialamplifier having one differential input circuit for receiving the pixeloutput and another differential input circuit for receiving a signalrepresenting the output of said differential amplifier, said anotherdifferential input circuit passing a current which mirrors the currentpassing through said one differential input circuit, wherein said firstcircuit comprises a first leg of said one differential input circuit ofsaid amplifier; a second circuit comprising a second leg of said anotherdifferential input circuit of said amplifier, said second leg passing acurrent which mirrors a current passing through said first leg, wheresaid second circuit mirrors a transistor configuration of said firstcircuit; and an enable circuit for enabling operation of said first andsecond legs.
 2. An imager readout circuit comprising: a first circuitprovided in at least one pixel for providing a pixel output; adifferential amplifier having one differential input circuit forreceiving the pixel output and another differential input circuit forreceiving a signal representing the output of said differentialamplifier, said another differential input circuit passing a currentwhich mirrors the current passing through said one differential inputcircuit, wherein said first circuit comprises a first leg of said onedifferential input circuit of said amplifier; a second circuitcomprising a second leg of said another differential input circuit ofsaid amplifier, said second leg passing a current which mirrors acurrent passing through said first leg, where said second circuitmirrors a transistor configuration of said first circuit; and an enablecircuit for enabling operation of said first and second legs.
 3. Theimager readout circuit of claim 2, wherein said first leg of said onedifferential input circuit is coupled to a positive input of saiddifferential amplifier, said second leg of said another differentialinput circuit is coupled to a negative input of said differentialamplifier.
 4. The imager readout circuit of claim 2, wherein said enablecircuit comprises: a node for receiving a supply voltage which suppliesa voltage to first and second legs; and a transistor coupled to a diodecircuit for selectively controlling said node.
 5. The imager readoutcircuit of claim 4, wherein said first and second legs are coupled to abias circuit.
 6. The imager readout circuit of claim 2, wherein saiddifferential amplifier further comprises: first and second branches,each branch comprising first, second, third and fourth transistors beingcoupled in series.
 7. The imager readout circuit of claim 6, wherein agate of said first, second, third and fourth transistors in said firstbranch is coupled to a gate of said first, second, third and fourthtransistors in said second branch respectively.
 8. The imager readoutcircuit of claim 7, wherein said gates of said first, second, and thirdtransistors in said first and second branches are coupled to arespective bias circuit.
 9. The imager readout circuit of claim 8,wherein said gates of said fourth transistors in said first and secondbranches are coupled between said second and third transistors in saidfirst branch.